Electric fuse with fusible element in form of a wire

ABSTRACT

An extremely compact electric fuse for small current ratings having a wire type fusible element whose length by far exceeds the length of the tubular casing. The fusible element forms two hair pin loops between the ends thereof and is surrounded by a sleeve of glass fibers normally preventing short-circuiting of contiguous sections of the fusible element in wire form and being so porous that it allows upon blowing of the fuse under major fault conditions, or short-circuit conditions, a substantially unimpeded flow of metal vapors through the aforementioned sleeve into a body of pulverulent arc-quenching filler inside the casing surrounding the sleeve.

United States Patent [1 1 Kozacka et al. v i

[111 3,748,622 July 24, 1973 1 ELECTRIC FUSE WITH FUSIBLE ELEMENT IN FORM OF A WIRE inventors: Frederick J. Kozacka, South Hampton, N.l-l.; Philip C. Jacobs, Jr., Newtonville, Mass.

[73] Assignee: The Chase-Shawmut Company,

Newburyport, Mass.

Filed: Dec. 7, 1972 Appl. No.: 312,920

US. Cl. 337/276, 337/273 Int. Cl. H0lh 85/18 Field of Search 337/273, 276, 295,

I References Cited UNITED STATES PATENTS [1/1950 Baxter 337/290 X 4/1958 Kozacka 337/273 X 7/l968 Jacobs 337/276 l2/l968 Smith et al 337/ 276 l/l969 Kozacka 337/222 X Primary Examiner-Bemard A. Gilheany Assistant Examiner-J E. Bell Attorney-Erwin Salzer 57 ABSTRACT major fault conditions, or short-circuit conditions, a

substantially unimpeded flow of metal vapors through the aforementioned sleeve into a body of pulverulent arc-quenching filler inside the casing surrounding the sleeve.

3 Claims, 3 Drawing Figures Patented July 24, 1973 Fig.

ELECTRIC FUSE WITH FUSIBLE ELEMENT IN FORM OF A WIRE BACKGROUND OF THE INVENTION The present invention relates to electric fuses particularly intended for the protection of transformers, which fuses have a current rating of a few amp s., e.g. 7 amps, and a voltage rating of about 600 volts, or more. Such fuses are needed for the protection of control transformers and in similar instances when it is imperative to minimize the size of fuses. This requirement coupled with the above voltage rating requirement calls for the provision of fusible elements in wire form whose length exceeds that of the casing, or fuse tube. This, in turn, calls for the provision of non-linear wires which are bent to appropriate shapes. This latter requirement is often a source of serious trouble because there is often a danger of short-circuiting at points where the fusible element or wire deviates from the straight path and the spacing between sections of the wire is small. This-danger is not limited to the fusible element or wire as such but makes itself felt mostly at the time when incident to blowing of the fusethe fusible element is replaced by an arc and a hot fulgurite surrounding the latter. At points where the spacing between points of the hot fulgurite is small there is a tendency of merger or short-circuiting of immediately adjacent portions of the fulgurite, resulting in such a large decrease of the resistance of the current path through the fuse immediately upon blowing thereof that the fuse fails.

It is the main object of this invention to provide fuses for transformer protection which are extremely compact, have fusible elements in wire form to achieve wire type time-current characteristics, are not subject to the above limitations and drawbacks and can be manufactured at relatively moderate cost.

The present invention makes use of the results of investigations by Philip C. Jacobs, Jr. disclosed in his US Pat.- No. I 2,833,890; May 6, 1958 for FILLERLESS ONE TIME FUSE.

SUMMARY OF THE INVENTION Fuses embodying this invention include a tubular casing of electric insulating material having a predetermined length. They further include a pair of metallic terminal elements at the ends of said casing. A fusible element in form of a wire having a length far in excess of the length of the casing is arranged inside of the easing and conductively interconnects said pair of terminal elements. The fusible element extends from one of the pair of terminal elements to a first point relatively close to the other of said pair of terminal elements and forms a first hair pin loop at said first point. The fusible element extends from said first point to a second point relatively close to said one of said pair of terminal elements, forming a second hair pin loop at said second point and extends from said second point to said other of said pair of terminal elements. The fusible element is surrounded along a preponderant portion of the length thereof by a relatively loosely woven porous sleeve of glass fibers including the region of said first loop thereof and the region of said second loop thereof. There is a pulverulent arc-quenching filler inside the casing surrounding the sleeve of glass fibers to cool the metal vapors which flow relatively freely through the pores of the sleeve.

BRIEF DESCRIPTION OF THE DRAWINGS some details of the structure of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION Reference numeral 1 has been applied in the drawings to indicate a tubular casing of electric insulating material, e.g. a laminate of glass-cloth-melamine. Casing 1 is closed at the ends thereof by metallic terminal elements in the form of caps 2a,2b. The fusible element 3 is in the form of a wire which is circular in crosssection. Wire 3 is arranged inside of easing l and conductively interconnects terminal caps 2a,2b. It extends from terminal cap 2a to a first point la relatively close to terminal cap 2b, forming a first hair pin loop at the point 1a and extends from the point la to a second point lb relatively close to terminal cap 2a, forming a second hair pin loop at the point lb andfurther extends from the point lb to terminal cap 2a. Reference numeral 4 has been applied to indicate a sleeve of glass fibers surrounding a preponderant portion of the length of fusible element 3 including the regions of points la and lb where the fusible element 3 forms hair pin loops. The pulverulent arc-quenching filler 5 inside of casing l surrounds the sleeve '4 of glass fibers. This sleeve has a relatively loose weave to render it porous to the passage of metalvapors. Filler 5 is preferably formed by quartz sand. Caps 2a,2b have axially inner portions 2a',2b' crimped radially inwardly into the outer surface of casing 1. Points la and 1b where wire 3 and sleeve 4 form loops are situated within the regions of easing l surrounded by the axially inwardly crimped portions 2a',2b' of terminal caps 2a,2b. .This geometry allows to maximize the length of the fusible element 3 for a given voltage rating and a given length of casing 1. It has been possible with the structure shown to evolve highly effective current-limiting transformer fuses for circuit voltages of 600 volts having a casing length of but 1% inches. These fuses are capable of clearing both major faults or short circuit currents and also overload currents.

Referring againto the drawings, the fusible element 3 in form of a wire projects through perforations in the convex end surfaces of caps 2a,2b from the inside of casing l to the outside thereof. The axially outer ends 4' of sleeve 4 are positioned immediately adjacentto the aforementioned perforations in caps 2a,2b.,While it is not absolutely necessary to make sleeve 4 entirely coextensive with wire 3, this is desirable, as will become more apparent from what follows.

The ends of the fusible wire 3 project initially substantially beyond sleeve 4, as clearly shown in FIG. 3. After the ends of wire 3 are threaded through the perforations in the end surfaces of caps 2a,2b the excess length of wire 3 is clipped or cut off and the residual ends of wire 3 projecting into the concave end surfaces of caps 2a,2b are conductively connected to the latter by solder joints 6.

Wire 3 may be of silver and may be provided in the center thereof with a silver-severing overlay (not shown) of a metal having a lower fusing point than silver, e.g. tin.

The function and operation of sleeve 4 depends upon the state the fuse structure is in.

As long as the fuse is carrying a current which does not exceed the particular current rating of the fuse, the sleeve 4 functions as an insulating means precluding short-circuiting of immediately adjacent portions of wire 3 whose spacing is extremely small.

n occurrence of major fault or short-circuit currents the entire length of wire 3 vaporizes virtually simultaneously which, in effect, is a violet explosion. Under such conditions the flow of the hot metal vapors through the pores of sleeve 4 is extremely rapid because the sleeve 4 offers a small resistance to the flow of these vapors substantially at right angles to wire 3 into the ambient arc-quenching filler 5. In such instances the arc-quenching filler 5 absorbs almost all of the are energy and its effect on the sleeve 4 is minimal, ie the latter undergoes very little or no changes, as a result of the explosion of wire 3. Because of this fact and because the venting area of sleeve 3 is relatively large per unit of length of wire 3, the whole body of filler 5 is allowed to fonn an effective cooling means for the freely escaping metal vapors The operation of sleeve 3 is again entirely different on occurrence of overload currents of excessive duration. Then wire 3 burns back from its center, sleeve 4 and is caused to absorb a relatively large amount of arc energy and tends to turn into a fulgurite, the outer layer of filler 5 operating as an aftercooler to rapidly reduce the temperature of the fugurite formed by sleeve 4. Because of the layers of filler 5 between immediately adjacent sections of sleeve 3 there is no danger that sections of sleeve 4 which turn into a fulgurite produce internal shon-circuiting of portions of the current path through the fuse structure.

FIG.3 shows parts 3 and 4 prior to complete assembly of the structure and FIG.] shows parts 3 and 4 in the ultimate positions thereof when the straight portions of parts 3,4 are substantially equidistant.

The axial escape of products of arcing out of sleeve 4 toward terminal caps 2a,2b is so small that no damage to caps 2a,2b has ever been observed.

We claim as our invention:

1. An electric fuse including a. a tubular casing of an electric insulating material having a predetermined length;

b. a pair of metallic terminal elements at the ends of said casing;

c. a fusible element in form of a wire having a length far in excess of said predetermined length of said casing arranged inside said casing and conductively interconnecting said pair of terminal elements, said fusible element extending from one of said pair of terminal elements to a first point relatively close to the other of said pair of terminal elements, forming a first hair pin loop at said first point and extending from said first point to a second point relatively close to said one of said pair of terminal elements, forming a second hair pin loop at said second point and extending from said second point to said other of said pair of terminal elements;

d. a sleeve of glass fibers having a relatively loose weave and being relatively porous surrounding a preponderant portion of the length of said fusible element including the region of said first loop thereof and the region of said second loop thereof; and

e. a pulverulent arc-quenching filler inside said cas- I ing surrounding said sleeve of glass fibers.

2. An electric fuse as specified in claim 1 wherein said pair of terminal elements is formed by a pair of caps mounted on said casing and having axially inner portions crimped radially inwardly into the outer surface of said casing and wherein said first point and said first loop and said second point and said second loop are arranged within regions of said casing surrounded by said radially inwardly crimped portions of said pair of caps.

3. An electric fuse as specified in claim 2 wherein said fusible element in wire form projects through perforations in said pair of caps from the inside of said casing to the outside thereof and wherein the axially outer ends of said sleeve of glass fibers are positioned immediately adjacent said perforations. 

1. An electric fuse including a. a tubular casing of an electric insulating material having a predetermined length; b. a pair of metallic terminal elements at the ends of said casing; c. a fusible element in form of a wire having a length far in excess of said predetermined length of said casing arranged inside said casing and conductively interconnecting said pair of terminal elements, said fusible element extending from one of said pair of terminal elements to a first point relatively close to the other of said pair of terminal elements, forming a first hair pin loop at said first point and extending from said first point to a second point relatively close to said one of said pair of terminal elements, forming a second hair pin loop at said second point and extending from said second point to said other of said pair of terminal elements; d. a sleeve of glass fibers having a relatively loose weave and being relatively porous surrounding a preponderant portion of the length of said fusible element including the region of said first loop thereof and the region of said second loop thereof; and e. a pulverulent arc-quenching filler inside said casing surrounding said sleeve of glass fibers.
 2. An electric fuse as specified in claim 1 wherein said pair of terminal elements is formed by a pair of caps mounted on said casing and having axially inner portions crimped radially inwardly into the outer surface of said casing and wherein said first point and said first loop and said second point and said second loop are arranged within regions of said casing surrounded by said radially inwardly crimped portions of said pair of caps.
 3. An electric fuse as specified in claim 2 wherein said fusible element in wire form projects through perforations in said pair of caps from the inside of said casing to the outside thereof and wherein the axially outer ends of said sleeve of glass fibers are positioned immediately adjacent said perforations. 